In electronic circuits designed to receive supply voltages at two input terminals herein called V+ and V-, the V+ terminal is intended to receive a higher (i.e., more positive) potential than the V- terminal in normal operation. Generally, the V+ terminal receives a positive voltage, relative to a reference ground, and the V- terminal receives a negative voltage; or both voltages may be of the same polarity but different in magnitude. In some cases, however, it is possible for the supply voltage connections to be reversed, for instance, by the attachment of a supply connector in reverse. Unfortunately, when the supply voltage to an electronic circuit is reversed, the electronic circuit may be damaged. Many types of circuits may be damaged by supply reversal, especially junction isolated integrated circuits.
FIG. 1 shows a partly isometric, partly cross-sectional view of a part of a typical prior art integrated circuit, which includes a p-type substrate 10, a buried layer 12 of n+ type material, an n-type epitaxial layer 14, an n+ type diffusion 20 formed in the epitaxial layer 14 to act as a contact, and p-type isolation bands 16. The buried layer 12, epitaxial layer 14 (enclosed within junction isolation bands 16) and diffusion 20 are commonly referred to as the "pocket". The pocket is typically biased as close to V+ as possible while the p-type isolation bands are biased at V-. The p-n junction formed between the pocket and the isolation bands 16 is thus reverse biased. If supply reversal occurs, this junction becomes forward biased and the circuit will be damaged. To prevent forward biasing of this junction in this case, the pocket is often connected to the V+ supply via a diode (not shown). When supply reversal occurs, the diode disconnects the pocket from the power supply and this junction is not forward biased. Unfortunately, some circuit elements formed in such n-type pockets require biasing the pocket closer to the V+ supply than a diode allows. Two examples of such circuit elements are thin film resistors and diffused resistors.
Also shown in FIG. 1 is a thin film resistor 18 formed on an oxide layer (not shown) on the upper surface of the epitaxial layer 14. With such a thin film resistor 18, if the pocket is biased more negative than the resistor, for example, by a diode offset, the electrical field generated from the voltage differential may cause long term corrosion of the thin film resistor 18. Therefore, a diode cannot be used to connect the pocket to the V+ supply. However, a direct connection between the pocket and the V+ supply will result in damage to the circuit when supply reversal occurs.
FIG. 2 shows a partly isometric, partly cross-sectional view of a typical prior art integrated circuit including a p-type diffusion resistor 22. The construction of this integrated circuit is otherwise generally similar to that of FIG. 1. With such a construction, the resulting p-n junction between the resistor 22 and the n-type pocket must remain reverse biased. Because a voltage differential of a diode offset would be sufficient to forward bias this junction (assuming the resistor is connected to the V+ supply), a diode cannot be used to connect the pocket to the V+ supply. However, a direct connection between the pocket and the V+ supply will result in damage to the circuit when supply reversal occurs.
Connection of n-type pockets directly to a positive (V+) supply terminal ordinarily is not a problem. However, for automotive circuits or other products which require protection against negative voltages on the positive supply terminal, connection of n-type pockets to the positive (V+) supply terminal can create problems. A reverse voltage causes the junction formed between the p-type isolation bands and substrate and the n-type pocket to be forward biased. This results in clamping of the positive supply terminal to within less than a volt of the negative supply terminal. In the case of automotive circuits, large currents are available during field decay events and when a battery is connected in reverse. The integrated circuit, or more likely the bond wires, will be completely destroyed by such a reverse connection of a battery or during field decay events.
Other types of integrated circuits can also be damaged by supply reversal. The present invention may be readily applied to such circuits as well.
Accordingly, it is an object of the present invention to provide a circuit construction for protective biasing of integrated circuit elements.
Another object of the invention is to provide a circuit means which provides transistor isolation and prevents injection of unwanted carriers when a negative voltage is applied to the positive supply terminal.
Yet another object of the invention is to provide a circuit means which protects integrated circuit elements from reverse supply connection, and in particular, protects automotive circuits from reverse connection of a battery.